This invention relates to a novel method for fabricating a glass CRT (cathode-ray tube) panel comprising a viewing window and peripheral sidewalls, and particularly to fabricating a panel which exhibits simultaneously both relatively high compressive surface stresses and relatively low compaction.
A typical process for fabricating a glass CRT panel includes pressing the panel from a gob of glass, inserting mask support studs in the sidewalls of the panel while the window of the panel is at about 500.degree. C. and then annealing the panel. The annealing step is done in two stages including a short initial stage and a longer final stage. The shorter initial stage of annealing relaxes all of the stresses with short decay times in the glass, for example, by soaking the panel (that is, maintaining the panel in an ambient) at about 500.degree. C. for about 35 minutes. The longer final stage of annealing cools the panel at a controlled rate that imparts to the panel a relatively uniform high compressive surface stress, but not so high and/or nonuniform as to break the panel. High compressive surface stress is desirable in order to compensate for structurally-induced tensile stresses that are later applied to the panel, particularly after the panel has been assembled into a CRT and the CRT is evacuated.
It has been observed that glass CRT panels ordinarily exhibit a degree of compaction, as determined by the procedure described below, of between 120 and 150 ppm (parts per million). Compaction is the shrinkage in size of the panel that results from the processing steps required for assembling the panel into an operable CRT. These processing steps include at least one heating step at about 425.degree. to 450.degree. C. for about one hour after a luminescent viewing screen has been deposited on the inside surface of the panel window. Excessive compaction after the screen has been deposited can cause faulty video color rendition by a shadow-mask-type CRT, which is the most popular design for a color television picture tube.
In order to reduce the compaction of glass CRT panels, some tube factories, after receiving the panels from the glass factory, reheat the panels to about 450.degree. C. for about an hour. This reheating requires extra equipment, floor space, inventory, fuel and time. It is desirable therefore to provide a method for fabricating a glass CRT panel with both high compressive surface stress and low compaction, without the additional reheating step. The method should be capable of being carried out at the glass factory with existing equipment, without resorting to special jigs or unusual techniques.